Secondary electron emitter and method of making it



J. H. DE BOER ET AL SECONDARY ELECTRON EMITTER AND METHOD OF MAKING ITFiled May 21, 1937 NICKEL CORE mam owes a b MAGNESIA 1 (moo/175p COATEDCARBON COATC'ORE w/n/ um 0F PURE 04/150 BY 2 IIEATINGEZELTRODEBYH/G/lFmzusucrmaucrlon 11v ANA TMflSP/IERE omrzmm EXPOSE CARBON/ZED 3 a COREr0 EXCESS OF 1;

MAGNESIUM VAPOR HEAT CARBON 6 04760 4 CORE T0 900 C. //V I VACUUM 8 YHIGH FREOUENC Y INDUCTION IN VEN TORS JAN HENDRIK DE 805R.

HAJO BRU/NING,

FR/TS PR4 KKE AND KAREL MAR/NUS l/ANGESSEL ATTORNEY.

Patented Feb. 7, 1939 V UNITED STATES PATENT OFFICE SECONDARY ELECTRONEMITTER AND METHOD OF MAKING IT Application May 21, 1937, Serial No.143,946 In Germany February 10, 1936 5 Claims.

This invention relates to a surface havini a high secondary electronemitting capacity and to a method of manufacturing such a surface.

Although it is generally sought to reduce to a minimum the secondaryemission of the electrodes of an electric discharge tube, for instanceof the grids and the anodes, there are cases in which it is justelectrodes having a high secondary. emission that are wanted, forexample, in electron amplifying tubes, the so-called "electronmultipliers, in which a very high amplification is sought to obtain.

For achieving a high secondary emission it is known to furnish surfaceswith a layer of caesium oxide and caesium absorbed thereo, which layeris sometimes mixed with other oxides or metal particles and may alsocontain caesium. However, such a layer can be prepared inside the tubeonly and its preparation requires many operations so that the risk offailure is comparatively great.

We have now devised a surface which has a very high secondary emissionand can be established in a very simple manner, while the preparationneed not be effected inside the tube, but can be carried out by aseparate operation, the surface being afterwards introduced intodischarge tubes and the like. A surface according to the inventionconsists of a pure carbon layer having a high adsorption power andhaving adsorbed to it one or more metals of the second,

principal group of the periodic system having an atom number above 4.

A surface according to the present invention may be established, forinstance, by coating a supporting body or electrode for example, ofnickel with a carbon layer and by applying on ,to said layer one or moreof the metals referred to. Particularly good results are achieved whenheating a metal surface in an atmosphere of a simple hydrocarbon,preferably acetylene, in a high frequency member. Thus, bydecomposition' of the hydrocarbon a pure carbon layer is formed on themetal surface, which layer eminently adsorbs alkaline earth metals.According to the invention one or more of the abovementioned metals arethen caused to be adsorbed from the vapour-phase by such a layer.

According to one very favorable method according to the invention asupporting body such as a nickel electrode is coated with a layerconsisting of one or more oxides or hydroxides of these metals,preferably with magnesium oxide, whereon carbon is precipitated by highfrequency heating in an atmosphere consisting of a simple hydrocarbon,the body thus obtained being subsequently heated for some time in a.high vacuum. The hydrocarbons may, for instance, be acetylene, methaneor the like. The heating operation after provision of the carbon may,for 5 instance, be effected at a temperature of 900 to 1000 C. Veryprobably the oxide is partly reduced thereby and the metal produced isadsorbed to the carbon.

Owing to the easy method of manufacture, l surfaces according to thepresent invention may advantageously be used in electric dischargetubes, more particularly in socalled electron multipliers.

It has been found that such a surface may have a very high secondaryemission which in many cases may be of the order of magnitude ofsecondary electrodes per primary electron.

The method according to the invention may be carried out as follows. l

A nickel electrode is coated with a layer of magnesium oxide, which maybe effected, for instance, by spraying. An electrode thus coated isintroduced into an acetylene atmosphere and then the electrode iscovered with a carbon layer by high frequency heating. This treatmentmay, for instance, take several minutes. After that the electrodetogether with other parts is introduced into the bulb of a dischargetube and then the tube is exhusted. After that the surface obtained isheated, for some moments to a temperature of about 900 C. which may alsobe effected by high frequency heating. It has been found that a surfacethus established permits a secondary emission to be obtained of theorder of magnitude of 6 secondary electrons per primary electron at avoltage of 500 volts.

In the accompanying drawing Figure 1 shows the steps of the methodaccording to the invention, and Figure 2 shows in greatly enlargedcross-section a portion of an electrode made in accordance with theinvention.

In Figure 1 the outer rectangles numbered I, 2, 3, and 4 represent thesuccessive steps of the method, rectangles 2 and 4 representing steps 4which are always used. The small rectangles inside rectangles l and 3represent alternative ways of carrying out the first and third steps.If, in step I the alternative a is adopted, and the core is clean oruncoated nickel, then in step 3 the alternative a of exposure tomagnesium vapor is followed. If, in step I the alternative I: isadopted, and the core is nickel with a magnesia coating, then in step 3the alternative b of proceeding directly to the final step is followed.55

prises a nickel core 5 on which is a layer 6 of pure carbon depositedon.the core by high frequency induction heating in an atmosphere ofsimple hydrocarbon, such as acetylene. The

. pure carbon layer contains or is impregnated with an alkaline earthmetal such as magnesium absorbed in the carbon layer as indicated by thesmall crosses I, and supplied to the carbon layer by exposure tovaporized magnesum, or by reductlon of the magnesia on the magnesiacoated core by the carbon.

What we claim is:

1. The method of making a surface having secondary electron emissivityseveral times unity which consists in coating a metal core body with a,coating of carbon by heating said body by high frequency inductionheating in an atmosphere of a simple hydrocarbon, metallizing thesurface of said coated body by depositing on it an alkaline earth metalfrom the vapor-phase, and then heating the metallized body in vacuum.

2. The method of making a surface having secondary electron emissivityseveral times unity which consists in coating a metal core body with alayer of carbon absorbent of alkaline earth metal by heating said bodyby high frequency induction to a high temperature in an atmosphere ofacetylene at substantially room temperature, exposing said carbon coatedbody in vacuum to an excess of vapor of an alkaline earth metal to causemetal from the vapor-phase to be adsorbed by said carbon coated body,and then heating said body by high frequency induction heating to about900 C.

3. The method of producing an electrode havin: secondary electronemissivity several times unity which comprises coating a body of nickelwith carbon highly absorbent of alkaline earth metal by heating saidbody to a high temperature in an atmosphere of acetylene by highfrequency induction heating, treating said carbon coated body at roomtemperature with an excess of magnesium vapor to impregnate said carbonwith magnesium, and heating said body in vacuum at about 900 C. forseveral minutes to activate said electrode.

4. The method of producing an electrode having secondary electronemissivity several times unity which comprises coating a body of nickelwith magnesium oxide, producing on said coated body=a layer of carbonhighly absorbent of alkaline earth metal by heating said body to a hightemperature in an atmosphere of acetylene by high frequency inductionheating, and heating said body in vacuum at about 900 C. for severalminutes to activate'said electrode.

5. A secondary electron emitting electrode having a ratio of secondaryelectron emission of several times unity upon impact by primaryelectrons at several hundred volts which comprises a metal core, a layerof alkaline earth oxide on and adherent to said core, and a pure carboncoating on and in contact with said layer and having the alkaline earthmetal of said layer absorbed in said carbon.

JAN HENDRIK or: BOER.

HAJO BRUINING.

FRITS PRAKKE.

KAREL MARINUS VAN GESSEL-

